1. Field of the Invention
The present invention concerns a circuit arrangement of the type that generates a reference signal using an oscillation generator.
2. Description of the Prior Art
In various electronic apparatuses and systems, high-stability and low-noise high-frequency (radio-frequency) signals are required as a reference. The function of the reference signal is dependent on the specific application case. For example, such a reference signal is used as a clock signal for analog-digital converters (ADC) and digital-analog converters (DAC) and as a clock signal for direct digital synthesis (DDS), and as a reference signal for phase control loops (PLL).
The quality (performance) of the signal with regard to stability or lack of noise is generally characterized via parameters such as, for example, the long-term frequency stability (deterioration due to aging), the short-term frequency stability, the frequency and phase stability dependent on the deterioration and/or temperature and/or microphony, the phase jitter or the single sideband phase noise power density (dBm/Hz or, with regard to the signal power, dBc/Hz) in different frequency intervals.
Due to their excellent properties (short-term stability, low carrier-proximal phase noise, low noise floor, low price), primarily quartz oscillators are used as signal sources for reference signals. To support a low sensitivity relative to temperature fluctuations, these are frequently operated in a regulated oven (oven controlled crystal oscillator, OCXO). The circuit and mechanical design represents a significant challenge for the developer and is additionally possible only in close cooperation with the quartz manufacturer. It can therefore be economically reasonable to obtain the complete OCXO as a standard component. In the circuit design, the measures necessary to achieve the individual requirements compete to a certain extent. For example, in quartz oscillators a minimum deterioration results due to the use of quartz grinding, but this that does not simultaneously represent an optimum with regard to a low phase noise. Under the circumstances, compromises must be made in the specification of the various parameters or a suitable OCXO that satisfies all specified values cannot be found at least (in the predetermined price range).
A further difficulty arises from the fact that quartz for this application can be produced only for frequencies up to a few hundred MHz (harmonic quartz). Its quality additionally severely decreases with increasing frequency.
The generation of extremely long-term stable and low-noise reference signals, as well as high-quality reference signals with frequencies greater than 100 MHz, is thus not possible with a conventional OCXO by itself. Rather, particular circuit-oriented measures are required.
Circuit arrangements of the aforementioned type are known from the text by Ulrich L. Rohde, “Microwave and Wireless Synthesizers—Theory and Design”, pages 106 through 110, published 1997. Three technically feasible solution possibilities are contrasted there for a reference frequency preparation for frequencies greater than 200 MHz, in the example a preparation for 640 MHz.
It is common to these approaches that an OCXO signal of 10 MHz is multiplied with a factor of n=64. A fundamental basis for the above considerations is the fact that the sideband noise and possible interfering signals (spurious signals) increase by the same factor n. A rise in dB by 20*log n results from this for the single sideband noise power density, a rise of 36 dB given a multiplication by 64. Given a multiplier chain, noise contributions of the individual elements have a greater effect the further forward in the chain that they are arranged. If very low-noise doublers are used, the noise of the OCXO dominates.
The single sideband noise power density (SSB noise power density) of an oscillator signal can be determined for predetermined frequency intervals by the Leeson equation (see, for example, the text by Peter Vizmuller, “RF Design Guide—Systems, Circuits, and Equations”, published 1995, page 230). As explained therein, the same connection exists between the oscillator frequency and the SSB noise power density as in the case of a frequency multiplication. A doubling of the oscillator frequency yields a rise of the SSB noise power density by 6 dB when all remaining parameters (transistor noise factor, oscillation circuit performance, signal power) are equal. As noted above, however, the quality of the quartz decreases with increasing frequency, and so in general more advantageous values with regard to the phase noise power are achieved via a multiplication of a relatively low initial frequency than by a direct generation at the end frequency.
In the aforementioned Rhode text, the possibility called architecture B is favored, but this exhibits a decisive disadvantage for some applications. Due to the extreme temperature responses for quartz filters, a rigid coupling of the phase of the 640 MHz signal to the 10 MHz signal is lost. Since the phase response of the appertaining filter scales with the multiplication factor of the following stages, a phase response of a 10 MHz filter immediately following the OCXO would have a particularly strong effect. One solution, in which the filter would be integrated into the oven, would produce a customer-specific and thus extremely expensive component as a result. If the filter is arranged further back in the chain for this reason, the filter effect decreases severely, meaning that the carrier-proximal noise is suppressed only insufficiently since, at high frequencies, technological limits exist as to the filter selection and the narrowband capabilities thereof. With increasing filter frequency, a compromise with regard to filter bandwidth and reliability of the arrangement must increasingly be made, since the filter transmission range can differ due to temperature response, deterioration or error compensation of the signal frequency.
Additionally, narrowband quartz filters present the risk of converting oscillations (for example air vibrations) into components of the electrical signal produces thereby. This behavior is designated as microphony.
A method for synchronization of a disrupted input signal with the associated transmission signal as well as a corresponding circuit arrangement is known from DE 302 92 49 C2. The method described therein is used for signals with a time-bandwidth product that is significantly larger than one (spread spectrum systems). The transmission signal is thereby regenerated from the disrupted acquisition signal by means of a phase-regulated filter. A correlation of the acquisition signal with the associated transmission signal after the occurrence of a start synchronization is by possible without conversion.